The present invention relates to a motor controller for spindle motors. In particular, the present invention relates to a digital motor controller having a dithered demand input to increase digital spindle control resolution.
The primary function of a spindle motor controller is to regulate the commutation of the spindle motor, which in turn controls the rotation rate of the spindle motor. This rotation rate is controlled by an external hard disk drive controller which provides the spindle motor controller with a digital demand code indicative of a desired rotational rate of the motor. It is often necessary to maintain very precise control over this rotational rate.
Digital spindle motor controllers offer several advantages over analog motor controllers. For instance, a digital controller is more easily programmed and adapted for use with a wide variety of motors and hard disk drive types. Digital controllers, however, often have limited resolution in their control over the rotational rate of the motor. This limited resolution results from the limited length of the digital words manipulated by the controller in providing drive signals to the motor. Limited resolution also results from having a limited system clock frequency from which to derive drive signals for the motor.
Particularly troublesome is a pulse width modulator used in generating pulse width modulated signals for driving the motor. In creating these signals, the pulse width modulator generally uses an up/down counter to first generate a triangle (or saw-tooth) shaped waveform. This waveform is formed essentially of digital words limited in their resolution by a ratio of the system clock used by the counter as a basis for generating the waveform to the frequency of the pulse width modulation. The pulse width modulator then compares individual digital words of this waveform to a digital word generated by multiplying a shape, or waveform, coefficient retrieved from a look up table and the digital demand code. Because rounding errors will necessarily result from this multiplication, control resolution is reduced because an increase in the digital demand code may not result in an increase in the rotational rate of the motor.
The resolution of the digital words forming the triangle (or saw-tooth) waveform can be increased by either increasing the system clock frequency or decreasing the frequency of the pulse width modulation. Both solutions, however, are undesirable. Increasing the clock frequency would increase power dissipation and/or complicate the design of the motor controller; for instance, a phase-locked loop may be required to generate the clock if the clock frequency is increased substantially. Decreasing the frequency of the pulse width modulation would increase the ripple in the current waveform, which could in turn decrease efficiency, increase distortion, and harm acoustic performance.